The advent of nanotechnology has brought about significant technological advancement in many fields of study. The birth of nanofluids (NFs) as advanced thermal fluids in the area of thermal management is a laudable and notable feat. NFs (mono and hybrid) have been extensively researched and proven to be better than conventional thermal fluids, and this is due to their enhanced thermophysical and convective properties ^{[1][2][3][4][5][6][7][8][9][10][11][12][13]}[1,2,3,4,5,6,7,8,9,10,11,12,13]. The application of diverse mono and hybrid NFs in various thermal systems has been studied experimentally ^[5][6][12][5,6,12] and numerically ^{[14][15][16][17]}[14,15,16,17], and found to improve heat transfer characteristics better than traditional thermal fluids. NFs have been investigated in the various types of convective heat transfer studies, such as natural ^{[18][19][20][21][22]}[18,19,20,21,22], mixed ^[23][24][23,24], and forced convection ^{[25][26][27][28][29][30]}[25,26,27,28,29,30] at laminar, turbulent, and transition regimes. These studies showed the enhancement of heat transfer performance with the HNFs found to be better thermal fluids than MNFs. In addition, the use of mono and hybrid NFs in heat transfer systems such as solar collectors ^{[12][31][32][33]}[12,31,32,33], radiators ^[34][35][36][34,35,36], refrigerators [37], mini-channel [38], microtubes ^[39][40][39,40], heat pipes ^[20][41][20,41], air-conditioning [42], heat exchangers [43], etc, have been studied. The deployment of NFs in these thermal transporting devices showed improvements in the heat transfer and flow characteristics than when conventional thermal fluids were used. Furthermore, mono and hybrid NFs/NPs have been employed as coolants (metal rolling process and metal machining operation) ^{[44][45][46][47]}[44,45,46,47], lubricants (automobile) ^{[48][49][50][51]}[48,49,50,51], thermal storage materials ^[52][53][54][52,53,54], sensors ^[55][56][57][55,56,57], drilling muds ^[58][59][58,59], chemically enhanced oil recovery material ^[60][61][62][60,61,62], etc, and are better thermal fluids/materials than the conventional thermal fluids/materials.

The suspension of diverse NPs into various base fluids to synthesize NFs has been proven to possess superior thermal properties compared with the traditional thermal fluids. Current research progress has revealed that the suspension of HNPs (mixing of two or more NPs) in different base fluids possessed better convective and thermal properties than MNFs ^[2][4][12][2,4,12]. MNF and HNF preparation appears to be a simple practice but complex in the true sense of it. Stability, which is the even distribution of mono and hybrid NPs in the base fluid, is key to the results associated with the thermal ^[63][64][63,64] and convective properties ^{[65][66][67][68]}[65,66,67,68] and performance ^{[68][69][70][71]}[68,69,70,71] of mono and hybrid NFs in various areas of their application. The stability of mono and hybrid NFs has been proven to significantly affect their thermal properties ^{[72][73][74][75][76][77][78][79][80][81]}[72,73,74,75,76,77,78,79,80,81] and convective heat transfer performances ^[65][82][83][65,82,83]. The instability is marked by sedimentation and agglomeration of the mono and hybrid NPs suspended in the base fluid. This consequently leads to inaccurate results when the resultant mono and hybrid NFs are deployed in different applications ^{[65][67][69][70][71][84][85]}[65,67,69,70,71,84,85]. This goes to show that obtaining good and desirable stability of mono and hybrid NFs is crucial. However, the stability of mono and hybrid NFs are strongly connected to preparation variables, such as stirring time, rate, temperature, sonication time, power, frequency, amplitude, and dispersion fraction (where a surfactant is used) ^{[7][75][77][81][86][87][88][89]}[7,75,77,81,86,87,88,89]. The sonication variables (time, power, mode, frequency, and amplitude) are related to the sonication energy required to achieve homogenized and stable mono and hybrid NFs ^{[74][78][90][91]}[74,78,90,91].

MNFs and HNFs are formulated through the suspension of MNPs and HNPs, respectively, into conventional thermal fluids, namely, base fluids, of which their stability is very important to the measurement of the thermophysical properties and convective studies. Fundamentally, MNFs and HNFs are formulated using a one- and two-step process (Figure 1). By this, the latter entails two processes, namely, (i) synthesis of MNPs or HNPs in the powdery form and (ii) suspension of MNPs or HNPs into the base fluids. The most reported process in the literature is the two-step process for the formulation of MNFs and HNFs, which can encourage their large-scale formulation at a low cost and an industrial utilization. The shortcoming of the two-step process relates to the sedimentation and agglomeration of MNPs and HNPs due to the Van der Waals forces of attraction among the particles [92]. The one-step process consists of the simultaneous production of MNFs and HNFs by way of synthesis and suspension of MNPs and HNPs in the base fluids. This technique is advantageous as it improves the homogeneity and stability of MNFs and HNFs and eliminates arduous procedures, such as drying and storing in comparison with the single-step process by reducing the agglomeration tendency of MNPs and HNPs ^[92][93][92,93]. However, the industrial application of this technique is impracticable except for low vapor-pressure fluids. This technique is also not cost-effective [94]. In addition, various one-step process techniques have been reported in the literature ^[93][95][96][93,95,96].

Numerous techniques have been reported in the literature for the characterization of MNFs and HNFs for their MNP and HNP shapes, sizes, distribution, functional groups, crystalline structure, surface morphology, dispersion, elemental composition, saturation, magnetization, etc. These techniques include Raman spectroscopy, X-ray diffractometer, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, vibrating sample magnetometer, energy-dispersive X-ray spectroscopy, light scattering, and transmission electron microscopy ^{[1][17][78][97][98][99][100][101]}[1,17,78,97,98,99,100,101]. The most used technique for characterizing MNFs and HNFs is transmission electron microscopy (TEM), followed by scanning electron microscopy (SEM) and the X-ray diffractometer (XRD). These most used techniques are often engaged as a stand-alone technique or with other techniques for MNF and HNF characterization. TEM is used to determine the size, shape, and dispersion of MNPs and HNPs in MNFs and HNFs, respectively, while the SEM detects surface morphology and elemental mapping. XRD is used to show the crystalline structure and grain size of MNPs and HNPs contained in MNFs and HNFs, respectively.

The suspension of MNPs and HNPs in various base fluids introduces charges into the base fluids, which leads to the formation of an electrical double layer (EDL) around the particle surface [102]. Therefore, MNFs and HNFs are referred to as electrically conducting fluids. By applying a potential across these fluids, oppositely charged electrodes tend to attract the MNPs or HNPs and EDL. The formation of EDL is strongly connected to the volume fraction, size, surface charge of the particles, and concentration of ions in the base fluids. The stability and even distribution of MNPs or HNPs in the base fluids are vital in the application of MNFs and HNFs because the thermophysical (mostly κ and μ) and optical properties, and the efficiency of the same are significantly related to the concentration of MNPs or HNPs in the suspension ^[103][104][103,104]. Improving the stability of MNFs and HNFs to reduce agglomeration and sedimentation with the two-step process has led to the utilization of four techniques, namely, ultrasonication, surfactant addition, surface modification, and pH control.

Sonication is one of the techniques deployed to obtain homogeneous mixtures of NPs suspended in selected base fluids. Several studies demonstrated that sonication affected κ, absorbance wavelength, μ, cluster size, surfactants, the diameter of CNTs, and particle size ^{[74][78][79][85][105][106][107]}[74,78,79,85,105,106,107]. For NFs, a sonication time spanning a few minutes to several hours has been documented. It can be deduced that an optimum sonication time (mainly due to the Brownian motion of MNPs or HNPs) occurred where the variable investigated either reduced (for μ and κ) or increased (for CNT diameter, particle, and cluster size). An optimum sonication time ranging from 12 min [108] to 60 h [105] has been reported in the literature for MNFs and HNFs. This reflects the need to optimize sonication time as it relates to other variables to achieve improved stability. However, this is mostly not the case for most of the studies on the formulation of MNFs and HNFs, except for very limited studies that have optimized the sonication parameters ^{[74][85][109]}[74,85,109]. Sonication of NFs has been reported to be carried out using the following three different types of ultrasonicators: probe-type, sonication-bath type, and shaker-type ^{[68][80][110]}[68,80,110]. Figure 2 shows the sonication of HNF.

Surfactants are complex chemical compounds that create an electrostatic repulsion to overcome magnetic attraction (for magnetic NPs) and Van der Waals interaction between NPs to avoid their sedimentation in the suspended base fluids [111]. The primary reason for surfactant use in NF formulation is to aid the stability of NPs in the base fluid [111]. Surfactants lower the interfacial tension between NPs and base fluid to enhance the stability of NFs. The use of surfactants promotes the stability of NFs by increasing the EDL between NPs. Surfactants such as cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS), gum Arabic (GA), oleic acid (OA), polyvinyl pyrrolidone (PVP), nanosperse AQ, dodecyl trimethyl ammonium bromide, sodium dodecylbenzene sulfonate (SDBS), and hexa decetyl trimethyl ammonium bromide have been used in the literature to stabilize MNFs and HNFs ^{[73][95][104][112][113][114][115]}[73,95,104,112,113,114,115]. A list of some surfactants used in NF studies is given in Table 1. An increase in κ, zeta potential (ZP), surface tension, and the μ of MNFs and HNFs due to the use of surfactants has been reported ^{[95][103][106][109]}[95,103,106,109]. However, the effectiveness of surfactants at >60 °C was reported to reduce due to weak bonds between surfactants and NPs, which, when finally broken, may lead to sedimentation and thus the instability of MNFs and HNFs [112]. Different surfactants have been used by various researchers to stabilize MNFs and HNFs formulated from diverse NPs and HNPs and suspended in different base fluids ^{[13][18][73][78][109][116][117]}[13,18,73,78,109,116,117]. Therefore, it can be concluded that the stability of MNFs and HNFs based on the use of surfactants is dependent on the type and nature (magnetic or not) of NPs or HNPs, the base fluid type (ionic or non-ionic), and the type of surfactants used.

121]. The pH of MNFs and HNFs is given in Table 32.

The stability of MNFs and HNFs can be improved by adjusting the pH. By suspending NPs into a base fluid, surface electric charges are produced on the resultant NF, which can be manipulated by altering the pH value. The surface electrostatic charges affect the stability of NF. An alteration of the pH value farther from the isoelectric point (IEP) enhances the NF stability. The pH of NF determines the IEP of the suspension, and this can be altered to improve the stability to avoid sedimentation and agglomeration. The IEP of some NFs is provided in Table 21. Additionally, the surface electric charge can be determined using ZP. The ZP measures the repulsion between NPs and increases with a rise in the particles suspended in the base fluid ^[73][118][73,118]. A high ZP (absolute value) indicates the stability of NFs due to a strong electrostatic repulsion between NPs, while a low ZP shows instability due to the weak electrostatic repulsion of particles. With a ZP value of >60 mV, a very stable NF is formulated; a value of >30 mV implies a stable NF, whereas <20 mV indicates weakness in NF stability ^{[73][96][103]}[73,96,103]. Zawrah et al. [73] reported the modification of the pH of Al₂O₃/water NF (with a surfactant of SDBS) from 5 to 10 using NaOH because the IEP of the NF was around 6.3. Similar pH alterations to improve NF stability were carried out in other studies ^{[85][106][119][120][121]}[85,106,119,120,

The surface modification or functionalization of NPs is another technique employed to improve the stability of NFs. This stability-enhancing method is surfactant-free but needs materials for functionalization. Although this technique is not widely studied, it is a promising method for the formulation of more stable MNFs and HNFs ^{[104][105][136]}[104,105,136]. Owing to the importance of the stability of MNFs and HNFs, the measurement of this parameter is key to the further use of MNFs and HNFs in terms of thermophysical properties and convective heat transfer studies.

The simplest method to check the stability of MNFs and HNFs is by visual inspection. In other words, it is a visual observation of the MNF and HNF samples at daily or weekly, or monthly intervals to see how the NPs or HNPs sediment with time. This is not a scientific method for checking the stability of MNFs and HNFs, as reported in the literature ^{[74][104][137][138]}[74,104,137,138]. However, this method is always used in addition to other stability monitoring techniques that are scientific ^{[74][104][109][137][138]}[74,104,109,137,138].

ZP is a method used to determine the stability of MNFs and HNFs. As earlier stated, the ZP of MNFs and HNFs is strongly connected to the repulsive force between the NPs or HNPs. This technique is mostly used to measure the stability of MNFs and HNFs, as reported in the literature by several authors ^{[1][73][98][121][131]}[1,73,98,121,131]. The degree of stability of MNFs and HNFs can be determined using this method based on the obtained ZP values. It is worth mentioning that this stability-checking technique is often used along with other techniques.

This method seems to be the most employed of all the methods for monitoring the stability of MNFs and HNFs. The absorbance or transmittance of the MNFs and HNFs at the peak wavelength can be deployed to monitor the stability of MNFs and HNFs ^{[20][79][85][109][139][140]}[20,79,85,109,139,140]. One distinguished merit of this method is the capability to check stability at regular intervals for a long time (days to months) [141], which other methods cannot offer. Thus, it provides an instantaneous measurement of the stability of MNFs and HNFs. Similar to other techniques, it is always used along with other methods such as visual inspection and ZP.

The stability of mono and hybrid NFs is also monitored by measuring their thermophysical properties over time. Garbadeen et al. [18] and Joubert et al. [20] monitored the stability of MWCNT/DIW and Fe₂O₃/DIW NFs for 250 min and 20 h, respectively, by measuring the μ. Likewise, in other studies, Yu et al. [142] and Ijam et al. [139] monitored the stability of Fe₃O₄/kerosene and GO/DIW-EG (60:40) NFs by measuring their κ for 360 min and 7 days, respectively. The use of κ to monitor the stability of NFs was corroborated by the work of Wang et al. [106], which reported a strong relationship between κ and the stability of NFs (Al₂O₃/W and Cu/W). Both Mahrood et al. [143] and Arani and Pourmoghadam [144] reported the use of density to monitor the stability of carboxymethyl cellulose-based Al₂O₃ and TiO₂ NFs (before and after) and EG-based Al₂O₃-MWCNT NF (five times in 14 days), respectively. Additionally, Babu and Rao [145] used turbidity to check the stability of water-based Al₂O₃ NF. The literature showed that two or more of these reported NF stability monitoring techniques were used to check the stability of mono and hybrid NFs.